Trucks commonly employ air brakes for stopping both the tractor and the trailer. These braking systems, relying on pressurized air, must include mechanical brakes on the tractor for use while the vehicle is parked. The mechanical brakes are typically spring brakes which normally tend to actuate the same brake shoes, through an actuator rod, as service brake chambers actuate. The spring brakes require application of air pressure to counteract the spring, and thus deactivate the brake, when driving. However, the braking systems found on trucks presently in use vary depending on when they were made and the duty requirements of the brakes. For example, some tractors have combined service and spring brakes on only one out of three axles. Since the force typically exerted by these spring brakes is about one-half of that which can be exerted by the service brake, the maximum braking power of such a tractor while parked is only one-sixth of that available when the vehicle is operating using the service brakes.
Parking brakes for trailers are of two basic kinds. One type, spring parking brakes, normally bias the actuator rod of the brake to a brake applying condition and are released only by applying air pressure to a spring brake chamber. These spring parking brakes used on trailers suffer from the shortcoming discussed above regarding spring brakes on tractors.
Trailers made in the U.S. before 1975 generally do not use spring parking brakes but typically use a relay emergency valve (REV) and an auxiliary tank of pressurized air which combine to maintain air pressure within the service brake chamber when the trailer is parked. The auxiliary tank is pressurized through the relay exhaust valve via a pressurized air supply line. These REV parking brake systems often leak so that after a few hours the braking force is substantially reduced. When the trailer is reconnected to a tractor pressurized air hose connections must be made before they are physically coupled so the auxiliary tank can be pressurized to set the brakes. The loss of pressure can be a safety hazard and increases the time it takes the operator to hook up to a parked trailer.
There have been numerous systems contemplated for using the service brake during emergency stops or parking. However, these prior art attempts have been generally commercially unsuccessful because they either fail to recognize existing legal requirements for braking systems, are too complicated, or are not readily adapted for retrofit to existing truck braking systems.